As a conventional AGC circuit, there is known a configuration in which an integrating circuit using a capacitor is utilized. This AGC circuit is provided with a variable gain amplifying circuit for amplifying or attenuating an input signal according to gain controlled by a gain control voltage to thereby output an output signal, a rectifier circuit for rectifying an output voltage of this variable gain amplifying circuit, an integrating circuit for integrating the voltage rectified by this rectifier circuit to convert it into a DC voltage, and a DC amplifying circuit for outputting a voltage proportional to a difference between the DC voltage inputted from this integrating circuit and a reference voltage, as a gain control voltage of the variable gain amplifying circuit. The integrating circuit is constituted by a resistor and a capacitor (refer to Patent Document 1).
Meanwhile, if a DC offset voltage is included in the input signal of the AGC circuit, the DC offset voltage affects an AGC operation adversely, and thus the AGC operation is not performed normally. Additionally, particularly in the audio systems, this DC offset voltage has an adverse influence on audibility, such as generating a certain noise or the like (It means a low-frequency sound that comes out of a loudspeaker in particular upon turning on and off the power. It may also be called a pop noise.). For that reason, an offset compensation circuit for compensating the DC offset voltage is required. There is known a configuration in which an integrating circuit using a capacitor in the offset compensation circuit is utilized (refer to Patent Document 2).
Moreover, particularly in the audio systems, a DC bias voltage of an output amplification stage connected to a subsequent stage of the AGC circuit for connecting external instruments, such as a line output stage, a headphone output stage, or the like may differ from a DC bias voltage of an audio signal processing stage including the AGC circuit. For that reason, a so-called bias shift circuit for shifting the DC bias voltage of the signal has been used.
Further, with enhancement in function and reduction in cost of audio equipment in recent years, demand for multi-channelization and reduction in the number of parts of the audio systems is increasing greatly.
Patent Document 1: Japanese Unexamined Patent Publication (Kokai) No. H8-116226
Patent Document 2: Japanese Unexamined Patent Publication (Kokai) No. H3-74913